Switch(3pm) Perl Programmers Reference Guide Switch(3pm)
NAME
Switch - A switch statement for Perl
VERSION
This document describes version 2.11 of Switch, released Nov 22, 2006.
SYNOPSIS
use Switch;
switch ($val) {
case 1 { print "number 1" }
case "a" { print "string a" }
case [1..10,42] { print "number in list" }
case (@array) { print "number in list" }
case /\w+/ { print "pattern" }
case qr/\w+/ { print "pattern" }
case (%hash) { print "entry in hash" }
case (\%hash) { print "entry in hash" }
case (\&sub) { print "arg to subroutine" }
else { print "previous case not true" }
}
BACKGROUND
[Skip ahead to "DESCRIPTION" if you don't care about the whys and
wherefores of this control structure]
In seeking to devise a "Swiss Army" case mechanism suitable for Perl,
it is useful to generalize this notion of distributed conditional
testing as far as possible. Specifically, the concept of "matching"
between the switch value and the various case values need not be
restricted to numeric (or string or referential) equality, as it is in
other languages. Indeed, as Table 1 illustrates, Perl offers at least
eighteen different ways in which two values could generate a match.
Table 1: Matching a switch value ($s) with a case value ($c)
Switch Case Type of Match Implied Matching Code
Value Value
====== ===== ===================== =============
number same numeric or referential match if $s == $c;
or ref equality
object method result of method call match if $s->$c();
ref name match if defined $s->$c();
or ref
other other string equality match if $s eq $c;
non-ref non-ref
scalar scalar
string regexp pattern match match if $s =~ /$c/;
array scalar array entry existence match if 0<=$c && $c<@$s;
ref array entry definition match if defined $s->[$c];
array entry truth match if $s->[$c];
array array array intersection match if intersects(@$s, @$c);
ref ref (apply this table to
all pairs of elements
$s->[$i] and
$c->[$j])
array regexp array grep match if grep /$c/, @$s;
ref
hash scalar hash entry existence match if exists $s->{$c};
ref hash entry definition match if defined $s->{$c};
hash entry truth match if $s->{$c};
hash regexp hash grep match if grep /$c/, keys %$s;
ref
sub scalar return value defn match if defined $s->($c);
ref return value truth match if $s->($c);
sub array return value defn match if defined $s->(@$c);
ref ref return value truth match if $s->(@$c);
In reality, Table 1 covers 31 alternatives, because only the equality
and intersection tests are commutative; in all other cases, the roles
of the $s and $c variables could be reversed to produce a different
test. For example, instead of testing a single hash for the existence
of a series of keys ("match if exists $s->{$c}"), one could test for
the existence of a single key in a series of hashes ("match if exists
$c->{$s}").
DESCRIPTION
The Switch.pm module implements a generalized case mechanism that
covers most (but not all) of the numerous possible combinations of
switch and case values described above.
The module augments the standard Perl syntax with two new control
statements: "switch" and "case". The "switch" statement takes a single
scalar argument of any type, specified in parentheses. "switch" stores
this value as the current switch value in a (localized) control
variable. The value is followed by a block which may contain one or
more Perl statements (including the "case" statement described below).
The block is unconditionally executed once the switch value has been
cached.
A "case" statement takes a single scalar argument (in mandatory
parentheses if it's a variable; otherwise the parens are optional) and
selects the appropriate type of matching between that argument and the
current switch value. The type of matching used is determined by the
respective types of the switch value and the "case" argument, as
specified in Table 1. If the match is successful, the mandatory block
associated with the "case" statement is executed.
In most other respects, the "case" statement is semantically identical
to an "if" statement. For example, it can be followed by an "else"
clause, and can be used as a postfix statement qualifier.
However, when a "case" block has been executed control is automatically
transferred to the statement after the immediately enclosing "switch"
block, rather than to the next statement within the block. In other
words, the success of any "case" statement prevents other cases in the
same scope from executing. But see "Allowing fall-through" below.
Together these two new statements provide a fully generalized case
mechanism:
use Switch;
# AND LATER...
%special = ( woohoo => 1, d'oh => 1 );
while (<>) {
chomp;
switch ($_) {
case (%special) { print "homer\n"; } # if $special{$_}
case /[a-z]/i { print "alpha\n"; } # if $_ =~ /a-z/i
case [1..9] { print "small num\n"; } # if $_ in [1..9]
case { $_[0] >= 10 } { print "big num\n"; } # if $_ >= 10
print "must be punctuation\n" case /\W/; # if $_ ~= /\W/
}
}
Note that "switch"es can be nested within "case" (or any other) blocks,
and a series of "case" statements can try different types of matches --
hash membership, pattern match, array intersection, simple equality,
etc. -- against the same switch value.
The use of intersection tests against an array reference is
particularly useful for aggregating integral cases:
sub classify_digit
{
switch ($_[0]) { case 0 { return 'zero' }
case [2,4,6,8] { return 'even' }
case [1,3,5,7,9] { return 'odd' }
case /[A-F]/i { return 'hex' }
}
}
Allowing fall-through
Fall-though (trying another case after one has already succeeded) is
usually a Bad Idea in a switch statement. However, this is Perl, not a
police state, so there is a way to do it, if you must.
If a "case" block executes an untargeted "next", control is immediately
transferred to the statement after the "case" statement (i.e. usually
another case), rather than out of the surrounding "switch" block.
For example:
switch ($val) {
case 1 { handle_num_1(); next } # and try next case...
case "1" { handle_str_1(); next } # and try next case...
case [0..9] { handle_num_any(); } # and we're done
case /\d/ { handle_dig_any(); next } # and try next case...
case /.*/ { handle_str_any(); next } # and try next case...
}
If $val held the number 1, the above "switch" block would call the
first three "handle_..." subroutines, jumping to the next case test
each time it encountered a "next". After the third "case" block was
executed, control would jump to the end of the enclosing "switch"
block.
On the other hand, if $val held 10, then only the last two "handle_..."
subroutines would be called.
Note that this mechanism allows the notion of conditional fall-through.
For example:
switch ($val) {
case [0..9] { handle_num_any(); next if $val < 7; }
case /\d/ { handle_dig_any(); }
}
If an untargeted "last" statement is executed in a case block, this
immediately transfers control out of the enclosing "switch" block (in
other words, there is an implicit "last" at the end of each normal
"case" block). Thus the previous example could also have been written:
switch ($val) {
case [0..9] { handle_num_any(); last if $val >= 7; next; }
case /\d/ { handle_dig_any(); }
}
Automating fall-through
In situations where case fall-through should be the norm, rather than
an exception, an endless succession of terminal "next"s is tedious and
ugly. Hence, it is possible to reverse the default behaviour by
specifying the string "fallthrough" when importing the module. For
example, the following code is equivalent to the first example in
"Allowing fall-through":
use Switch 'fallthrough';
switch ($val) {
case 1 { handle_num_1(); }
case "1" { handle_str_1(); }
case [0..9] { handle_num_any(); last }
case /\d/ { handle_dig_any(); }
case /.*/ { handle_str_any(); }
}
Note the explicit use of a "last" to preserve the non-fall-through
behaviour of the third case.
Alternative syntax
Perl 6 will provide a built-in switch statement with essentially the
same semantics as those offered by Switch.pm, but with a different pair
of keywords. In Perl 6 "switch" will be spelled "given", and "case"
will be pronounced "when". In addition, the "when" statement will not
require switch or case values to be parenthesized.
This future syntax is also (largely) available via the Switch.pm
module, by importing it with the argument "Perl6". For example:
use Switch 'Perl6';
given ($val) {
when 1 { handle_num_1(); }
when ($str1) { handle_str_1(); }
when [0..9] { handle_num_any(); last }
when /\d/ { handle_dig_any(); }
when /.*/ { handle_str_any(); }
default { handle anything else; }
}
Note that scalars still need to be parenthesized, since they would be
ambiguous in Perl 5.
Note too that you can mix and match both syntaxes by importing the
module with:
use Switch 'Perl5', 'Perl6';
Higher-order Operations
One situation in which "switch" and "case" do not provide a good
substitute for a cascaded "if", is where a switch value needs to be
tested against a series of conditions. For example:
sub beverage {
switch (shift) {
case { $_[0] < 10 } { return 'milk' }
case { $_[0] < 20 } { return 'coke' }
case { $_[0] < 30 } { return 'beer' }
case { $_[0] < 40 } { return 'wine' }
case { $_[0] < 50 } { return 'malt' }
case { $_[0] < 60 } { return 'Moet' }
else { return 'milk' }
}
}
(This is equivalent to writing "case (sub { $_[0] < 10 })", etc.; $_[0]
is the argument to the anonymous subroutine.)
The need to specify each condition as a subroutine block is tiresome.
To overcome this, when importing Switch.pm, a special "placeholder"
subroutine named "__" [sic] may also be imported. This subroutine
converts (almost) any expression in which it appears to a reference to
a higher-order function. That is, the expression:
use Switch '__';
__ < 2
is equivalent to:
sub { $_[0] < 2 }
With "__", the previous ugly case statements can be rewritten:
case __ < 10 { return 'milk' }
case __ < 20 { return 'coke' }
case __ < 30 { return 'beer' }
case __ < 40 { return 'wine' }
case __ < 50 { return 'malt' }
case __ < 60 { return 'Moet' }
else { return 'milk' }
The "__" subroutine makes extensive use of operator overloading to
perform its magic. All operations involving __ are overloaded to
produce an anonymous subroutine that implements a lazy version of the
original operation.
The only problem is that operator overloading does not allow the
boolean operators "&&" and "||" to be overloaded. So a case statement
like this:
case 0 <= __ && __ < 10 { return 'digit' }
doesn't act as expected, because when it is executed, it constructs two
higher order subroutines and then treats the two resulting references
as arguments to "&&":
sub { 0 <= $_[0] } && sub { $_[0] < 10 }
This boolean expression is inevitably true, since both references are
non-false. Fortunately, the overloaded 'bool' operator catches this
situation and flags it as a error.
DEPENDENCIES
The module is implemented using Filter::Util::Call and Text::Balanced
and requires both these modules to be installed.
AUTHOR
Damian Conway (damian@conway.org). The maintainer of this module is now
Rafael Garcia-Suarez (rgarciasuarez@gmail.com).
BUGS
There are undoubtedly serious bugs lurking somewhere in code this funky
:-) Bug reports and other feedback are most welcome.
LIMITATIONS
Due to the heuristic nature of Switch.pm's source parsing, the presence
of regexes with embedded newlines that are specified with raw "/.../"
delimiters and don't have a modifier "//x" are indistinguishable from
code chunks beginning with the division operator "/". As a workaround
you must use "m/.../" or "m?...?" for such patterns. Also, the presence
of regexes specified with raw "?...?" delimiters may cause mysterious
errors. The workaround is to use "m?...?" instead.
Due to the way source filters work in Perl, you can't use Switch inside
an string "eval".
If your source file is longer then 1 million characters and you have a
switch statement that crosses the 1 million (or 2 million, etc.)
character boundary you will get mysterious errors. The workaround is to
use smaller source files.
COPYRIGHT
Copyright (c) 1997-2006, Damian Conway. All Rights Reserved.
This module is free software. It may be used, redistributed
and/or modified under the same terms as Perl itself.
perl v5.10.0 2007-12-18 Switch(3pm)
Mac OS X 10.6 - Generated Thu Sep 17 20:15:27 CDT 2009